I see this sentiment a lot and don't really understand it. Until the last few books there is a very clear distinction between the human tech (hard SF) and the alien tech (magic). The main story takes place very much within the limits of plausible physics (ships rely on reaction engines and takes weeks to traverse large distances, space combat and weapons are actually realistic, powered armor doesn't keep people safe from inertia, etc.), and then there are magical elements that the characters run up against.
Contrast that with Three Body where the main plot driver revolves around FTL communication with indestructible intelligent nano-scale machines that can be anywhere any time, then gets progressively less realistic from there. I did enjoy the thread about the human ships going extra-solar, though, which was closer to Expanse in terms of Hard SF meeting a magical element (+dimensional travel).
What really irked me about The Three Body Problem was the way it blithely introduced faster-than-light communication as a plot point -- a key plot point! -- without any hint of a nod to the fact that this is impossible as far as we currently know. In fact, quite the opposite, the speed of light is taken to be a hard limit elsewhere in the story and that plays a key role too. So which is it?
I think it can be okay to invent magical tech in hard SF, if and only if you clearly distinguish the magical bits and work through all the ramifications logically. A great example is the “bobbles” in Vernor Vinge’s The Peace War and Marooned in Realtime.
TBP is also very cavalier about basic interactions with energy. To make a sophon, I kid you not, the aliens unfold a proton into a 2D sheet that they wrap around their entire planet. The unfolded proton has the same mass as the folded one (explicitly stated), but far from being invisible, it actually blocks all incoming light from their sun. That's orders of magnitude more energy than it takes to shatter protons in particle accelerators, smashing into the sophon for days on end, and it just... holds together? And then it gets to Earth and can apparently move and fold and unfold on its own and intercept all of our communications (which should also shatter it).
Contrast that with Three Body where the main plot driver revolves around FTL communication with indestructible intelligent nano-scale machines that can be anywhere any time, then gets progressively less realistic from there. I did enjoy the thread about the human ships going extra-solar, though, which was closer to Expanse in terms of Hard SF meeting a magical element (+dimensional travel).